In the gas assisted injection molding process a quantity of resin is injected into a mold by conventional means and a gas such as nitrogen is injected into the resin melt under pressure to fill out the mold cavity with the resin, thereby creating a hollow body portion within the resin. The gas is held under a pressure to press the resin against the mold cavity while the resin is cooling and solidifying. After the resin has solidified sufficiently, the gas pressure is vented and the mold is thereafter opened and the part removed.
Numerous methods and apparatus have been devised to accomplish the gas assisted injection molding process. These prior art methods and apparatus include arrangements whereby the gas enters the resin melt from the resin injection nozzle, whereby the gas enters the resin melt through the sprue bushing, whereby the gas enters the resin melt through the runner system, or whereby the gas enters the resin melt directly into the mold cavity. The prior art methods and apparatus also propose various means of venting the gas to atmosphere following the solidifying operation.
To simplify the method and the apparatus it is desirable to inject the gas into the resin melt by means of the resin injection nozzle so that a separate gas injection device is not required. It is also desirable for the sake of simplicity to vent the gas from the mold cavity through the resin nozzle following the molding operation. However, although the use of a single nozzle to inject both the resin and the gas into the mold cavity and to thereafter vent the gas from the mold cavity is desirable from a simplicity standpoint, there have been problems in the prior art in attempting to design such a nozzle that would consistently produce high quality parts and long term, low maintenance operation. For example, gas and resin tend to mix within the injection nozzle during the process with the result that surface blemishes may be produced on the molded part. Further, during the venting of the gas back through the injection nozzle, resin is often drawn back with the gas into the gas passageway with the result that the gas passageway is blocked and must be cleaned. There have, in turn, been various attempts to address these problems. Some prior art attempts involve complex shutoff nozzles in combination with gas passageways. Not only are these devices complex, but they also continue to provide an area within the nozzle where the resin and gas can mix so that the resin can enter the gas passageway during the venting step and cause a blockage of the gas passageway and/or cause surface blemishes on the next part to be molded. These devices are also typically relatively large in size and may require modification of the associated injection molding machine. Because of their size, they may also require modification of the devices to adapt to different molds. In any event, these devices do not solve the problem of mixing or blockage and can be a source of high maintenance and downtime.
In other prior art attempts to address these problems, the resin is deliberately allowed to flow into the gas passageway during the venting operation with the intent that the resin will be blown out of the gas passageway and back through the gas aperture during the next molding cycle. However, the resin expulsion is often incomplete with the result that the resin remaining will degrade with heat and time and block the device. The degraded resin may also contaminant the part. In addition, this contaminant resin may be drawn back into the gas passageway upon venting, thereby speeding up the blockage.
In other prior art attempts to address these problems, needles have been used that project into the sprue bushing. Such needles are delicate in nature and are subject to distortion, blockage and breakage under the heat and pressure encountered in normal production cycles.